Since the invention of the integrated circuit, the semiconductor industry has experienced continual rapid growth due to continuous improvements in the integration density of various electronic components (i.e., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from repeated reductions in minimum feature size, allowing for the integration of more components into a given area.
These integration improvements are essentially two-dimensional (2D) in nature, in that the volume occupied by the integrated components is essentially on the surface of the semiconductor wafer. Although dramatic improvements in lithography have resulted in considerable improvements in 2D integrated circuit formation, there are physical limits to the density that can be achieved in two dimensions. One of these limits is the minimum size needed to make these components. Also, when more devices are put into one chip, more complex designs are required.
An additional limitation comes from the significant increase in the number and length of interconnections between devices as the number of devices increases. When the number and length of interconnections increase, both circuit resistance-capacitance (RC) delay and power consumption increase.
Three-dimensional integrated circuits (3D IC) are therefore created to resolve the above-discussed limitations. In a conventional formation process of 3D IC, two wafers, each including an integrated circuit, are formed. The wafers are then bonded with the devices aligned. Deep vias are then formed to interconnect devices on the first and second wafers.
Much higher device density has been achieved using 3D IC technology, and up to six layers of wafers have been bonded. As a result, the total wire length is significantly reduced. The number of vias is also reduced. Accordingly, 3D IC technology has the potential of being the mainstream technology of the next generation.
Conventional methods for forming 3D IC also include die-to-wafer bonding, wherein separate dies are bonded to a common wafer. An advantageous feature of the die-to-wafer bonding is that the size of the dies may be smaller than the size of chips on the wafer.
Recently, through-silicon-vias (TSVs), also referred to as through-wafer vias, are increasingly used as a way of implementing 3D IC. Conventionally, a bottom wafer is bonded to a top wafer. Both wafers include integrated circuits over substrates. The integrated circuits in the bottom wafer are connected to the integrated circuits in the wafer 4 through interconnect structures. The integrated circuits in the wafers are further connected to external pads through through-silicon-vias. The stacked wafers can be subjected to a sawing process to provide a plurality of stacked die structures.